Other reductive couplings

Reductive indium-mediated coupling of aldimines obtained from aromatic aldehydes and aromatic amines generates vicinal diamines in aqueous ethanol (Eq. 4.82). °° Small indium rods were used in this study. The presence of NH4CI was found to accelerate the reaction and no side product due to unimolecular reduction was observed. The reaction fails completely in CH3CN, DMF, or wet DMF. The use of nonaromatic substrates also resulted in the failure of the reaction. 

Many transition metal-catalyzed cross-couplings such as the Stille coupling, the Heck reaction, the Suzuki reaction, and the Trost-Tsuji reactions have been carried out in aqueous conditions.In oiu own studies, we recently discovered that various palladium-catalyzed couplings of the above types can be performed in air and water. For example, a highly 

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Kagan and co-workers pioneered the work on the reductive behavior of the low oxidation states of the lanthanide elements in organic synthesis [2b]. Ln metals and Ln(II) derivatives were subsequently found to promote a number of important individual reactions [301]. The combination of one- and two-electron chemistry sets Sml2 apart from virtually every other reductive coupling agent currently available and exhibits exceptional properties for sequential conversions tolerating unprotected functional groups [lb]. 

Other syntheses of oxiranes which are occasionally useful and involve a species (64) are the dehydration of 1,2-diols (Scheme 69) (78TL5153, 7808(58)12) and the reductive coupling of aldehydes (Scheme 70) (B-73MI50500). 

This program on the 40th year of plutonium chemistry reminds us anew that plutonium has an exceedingly complicated chemistry. The near identity of its oxidation/reduction couples makes all oxidation states accessible and provides a wealth of chemistry not exceeded in any other element. 

An approach to the preparation of asymmetrically 1,2-disubstituted 1,2-diamines has been reported the zinc-copper-promoted reductive coupling of two different N-(4-substituted)phenyl aromatic imines, one bearing a 4-methoxy and the other a 4-chloro substituent, in the presence of either boron trifluoride or methyltrichlorosilane, gave a mixture of the three possible 1,2-diamines, where the mixed one predominated [31 ]. Low degrees of asymmetric induction were observed using 1-phenylethylamine, phenylglycinol and its 0-methyl ether, and several a-amino acid esters as the chiral auxiharies meanwhile the homocoupling process was not avoided (M.Shimizu, personal communication). 

The reduction by Ag" is the most widely investigated example although in recent years several other reductants have been used. The series of oxidations effected by the Ag -SjOg couple are referred to in the section on Ag(II) and Ag(III). In general the rate of disappearance of persulphate is independent of the concentration and, to some extent, the nature of the substrate, viz. 

As reductions by metals often occur by one-electron transfers, radicals are involved as intermediates. When the reaction conditions are adjusted so that coupling competes favorably with other processes, the formation of a carbon-carbon bond can occur. The reductive coupling of acetone to 2,3-dimethylbutane-2,3-diol (pinacol) is an example of such a reaction. 

Under other conditions, reduction leads to diols. Reductive coupling to diols can be done using magnesium amalgam248 or zinc dust.249... 

A number of chiral monodentate phosphines have been examined in asymmetric nickel-catalyzed reductive couplings of aldehydes and alkynes. The best results to date have been obtained with (+)-NMDPP (16) [33]. Aromatic internal alkynes and branched aldehydes participate with excellent enantiose-lectivity (Scheme 15), although yields and enantioselectivities were somewhat lower with other combinations of aldehydes and alkynes. In a complemen-... 

Most of the reactions that involve significant fractionation of Se and Cr isotopes appear to be far from chemical or isotopic equihbrium at earth-surface temperatures. Redox disequilibrium is common among dissolved Se species. Dissolved Se(IV) and solid Se(0) are often observed in oxic waters despite their chemical instability (Tokunaga et al. 1991 Zhang and Moore 1996 Zawislanski and McGrath 1998). In one study of shallow groundwater, Se species were found to be out of equilibrium with other redox couples such as Fe(III)/Fe(II) (White and Dubrovsky 1994). The kinetics of abiotic Se(VI) reduction, like those of sulfate, are quite slow. In the laboratory, conversion of Se(VI) to Se(IV) requires one hour of heating to ca. 100°C in a 4 M HCl medium. 

Lund and coworkers [131] pioneered the use of aromatic anion radicals as mediators in a study of the catalytic reduction of bromobenzene by the electrogenerated anion radical of chrysene. Other early investigations involved the catalytic reduction of 1-bromo- and 1-chlorobutane by the anion radicals of trans-stilhene and anthracene [132], of 1-chlorohexane and 6-chloro-l-hexene by the naphthalene anion radical [133], and of 1-chlorooctane by the phenanthrene anion radical [134]. Simonet and coworkers [135] pointed out that a catalytically formed alkyl radical can react with an aromatic anion radical to form an alkylated aromatic hydrocarbon. Additional, comparatively recent work has centered on electron transfer between aromatic anion radicals and l,2-dichloro-l,2-diphenylethane [136], on reductive coupling of tert-butyl bromide with azobenzene, quinoxaline, and anthracene [137], and on the reactions of aromatic anion radicals with substituted benzyl chlorides [138], with... 

The overall mechanism is closely related to that of the other cross-coupling methods. The aryl halide or triflate reacts with the Pd(0) catalyst by oxidative addition. The organoboron compound serves as the source of the second organic group by transmetala-tion. The disubstituted Pd(II) intermediate then undergoes reductive elimination. It appears that either the oxidative addition or the transmetalation can be rate-determining, depending on reaction conditions.134 With boronic acids as reactants, base catalysis is normally required and is believed to involve the formation of the more reactive boronate anion.135... 

The 1,2,3,4-isomer is also the major product if other reduction reagents [7] such as anhydrous hydrazine [6], diimide [3] or palladium hydride wrapped in gold foil [15] are used. Contrary to this result, reduction with wet Zn/Cu couple (Section 5.2.2) does not lead to the cis-l-adduct. Instead the e-isomer and the trans-3-isomer are formed as major products [5]. 

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